1994 Suburban - Chasing a Ghost

Discussion in 'Chevy Truck Forum' started by DrvLikHell, Jun 19, 2015.

  1. DrvLikHell

    DrvLikHell Full Member

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    Greetings and Sanitations!

    This forum seems like one of the best out there for knowledge and stuff so I registered to ask for help with my newly acquired truck. My background is mostly in import cars. Don't automatically think Honda or ricer, more like 3000GT VR-4, Rx-7, Conquest, BMW, etc. I don't do FWD stuff. I also used to have a Trans Am and a Cadillac, though I never needed to work on either one. I'm usually fairly decent at troubleshooting and fixing problems, but this big red beast has me boggled.

    I have a 1994 Chevy Suburban K1500 with 166,000 miles on the original 5.7. The only "mods" are aftermarket wheels, cat delete, and some kinda Flowmasterish muffler with chrome pipes out back. Otherwise it's all stock, except for repairs by previous owners, of which there have only been two (owners, not repairs).

    The truck starts and idles fine... most of the time. When applying throttle it bogs out and acts like it's missing, stuttering, etc. Going very lightly on the throttle will usually incur no problems, usually, not always. Sometimes it wants to die as soon as I touch the throttle, but that's rare. Sometimes I can give it more throttle before it bogs, and sometimes much less. 80% of the time it will idle perfect, the other 20% of the time it's a little rough, and once in a great while will stall at idle. Usually stalling at idle occurs when coming to a stop after being driven a short distance (as in around the block, it's not road worthy yet). These problems get worse when it's hot. The check engine light works but does not light up unless I unplug something that it wants me to leave plugged in. It does not throw a code when it bogs or stalls.

    The previous owner replaced the fuel pump before giving up and selling the truck to me. I replaced both engine temperature sensors. Then I replaced the MAP sensor after testing led me to believe that it was bad. After replacing the MAP sensor I discovered that the MAP sensor connector had two of the wires in the wrong places on the plug. The ground and data wires were swapped on the plug, so I put them back where the diagram says they're supposed to go. After that, both MAP sensors tested correctly, so the original MAP wasn't bad in the first place. No idea how the two MAP connector wires got swapped. I tested the TPS which checked out fine. I also tested the EGR solenoid for leaks and it does not leak vacuum at idle. There is good vacuum going to it. When I got the truck, the EGR valve was not connected to the EGR solenoid. The EGR solenoid had it's vacuum output line looped back onto it's vent. It is now connected to the EGR valve again. I sorta tested the IAC by unplugging it and starting the engine. It idled just above 1500 rpm and then slowly went back down to normal after I plugged it back in. It threw an IAC code while doing this which I later cleared.

    In observation, it smells like it's running very rich and when revving the engine with the air cleaner off it will sometimes shoot flame out of the TBI as it chokes and sputters. It only does that when I apply throttle somewhat aggressively. Otherwise it only chokes and sputters with no fireworks. The previous owner was well versed in carburetors, not so much in fuel injection, and he said that if it was a carburetored engine he would say that it's running as if the choke is stuck shut. I checked 5 out of the 8 sparkplugs and, so far, only number 3 looked like it was experiencing a rich condition. It was black and looked as if it had been damp. The other 4 I looked at were "chocolate" at the insulator with white/grey at the tip of the ground electrode. I didn't check number 6, 7, or 8 yet because it's 95+ degrees where I am. The distributor cap looks fairly new but had some crustyish ash residue on each of the terminals on the inside. I scraped this off and it didn't make a difference. Not sure if that's a really bad thing because I'm not used to distributors.

    Next on my list is to check the EGR valve itself. Someone said to squeeze it and see if it affects the engine. Odd, but I've squeezed stranger things before. Alternatively, I think I'll apply vacuum to it and see if there's a difference in how the engine runs. The fire out of the TBI makes me think timing, so I'm going to acquire a timing light and check that later this week. I will also check the fuel injector spray cones later when my driveway won't cook an egg. If those last three things check out... I'm lost.

    If anyone has any other ideas on things to check, please let me know. If these last three things check out, I have no idea what to look at next. I may try to build an OBD1 serial cable to hook it up to my laptop to watch what goes on in real time with WinALDL, but that might not give any insight either. The Autozone manager said I'm chasing a ghost and I'm starting to think he's right.


    -Mike
     
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  2. Bad88

    Bad88 Full Member

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    Hello & welcome Drvlikhell.

    No ghosts. Just hang in there and do not give up & you'll get the problem(s) solved.

    It does sound like a rich condition from what you described. I always suggest doing a good visual inspection before undertaking any hands on tasks. Look for missing, loose, or cracked vacuum lines. It may also pay off for you to verify the rest of the sending unit / sensor wires since you all ready found one with the wires reversed! Once you feel comfortable that those things are in order then move on.

    I would pull the other spark plugs and check them. I would be inclined to replace them anyway and I would install a new cap and rotor while you're at it.

    I suspect two things: 1st - sounds like the timing may be off a little. 2 - it is possible that the TBI needs to be cleaned and rebuilt with fresh gaskets & O-rings, and new fuel screens at the base of each injector (near the spray nozzle).

    To me, that is the best place to start (after a visual inspection) since you have a unfamiliar vehicle and you are going by what the previous owner has told you. Once you know the tune up is good, and the timing is correct, then you can begin to recheck sensors, etc. In my mind, checking sensors and getting codes (without a tune up) could be from issues with the items I suggested you do (tune up, etc.). Eliminate the easy stuff and go from there. Don't rule out the TPS or anything else just yet. Sometimes one sender may test faulty but in reality it is processing based on bad input (if that makes sense).

    That said; there are some REALLY smart guys on this forum who can expertly guide you in the codes & sensor area.
     
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  3. chevychase

    chevychase Semi-Admin
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    Check make sure distributor is not loose..check gasket at the base of throttle body ..spray carb. cleaner around it..rpm,s will increase if bad.
    Do this to all vac. lines...look at the injectors spray pattern..should be a nice fan /coned shape pattern..not bubbles..or erratic.
    Check timing as said..set at 0 degrees...have to disconnect timing tab/line..tan wire black stripe..under carpet area..passenger side floor board..on a 94.
    If not there will be below black cover says..fuel relay..fuse..junction.
    click here for timing info.. could try this disconnect egr see if it changes things..also map...will get a service light..can clear..pull ecm/ign..fuse for 30 seconds.
    I would check fuel pressure also click here. good starting place..welcome to the forum..yes this is best site for help!!
     
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  4. scavanger

    scavanger Full Member

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    I have used WinALDL but been about three yrs ago. I bought a USB cable online , works great. If I remember right it gives you O2 sensor voltage readings. I had a 95 that gave me some of the problem you have,I did notice the readings being at the low end I think it was , didn't say it was bad but I think it was a problem. When I hit deep puddles of water with the right side it would stumble, lose power and in general run bad for about 3 to 5 min. I checked every wire connection etc and found nothing. I think the sensor may have been getting wet cooling it off enough that it went out of range .I never did get to verify my thoughts on that by replacing the O2 sensor cause I sold the truck.
     
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  5. DrvLikHell

    DrvLikHell Full Member

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    Thanks for the quick replies!

    I finished checking the plugs and they all look the same as the others (excluding #3). I rechecked #3 and it appears to be returning to a normal condition with a lighter area at the tip of the ground electrode, though still not quite like the others. I squeezed the EGR valve and the idle decreased. Letting go returned the idle to normal so I assume the EGR is working, or at least not leaking exhaust gas into the intake at idle. Since the EGR valve was disconnected when I got the truck, and plugging it back in hasn't made a difference, I suppose I can rule out EGR problems.

    I did previously disconnect the MAP sensor as a test. It started and ran like garbage until the check engine light came on after a second or two, at which time it cleared right up. It almost seemed to idle a little smoother without the MAP, but it drove pretty much the same, possibly worse. It was similar enough so that it was hard to tell if it was the absence of the MAP sensor or if it was just getting worse as the engine warms up, which is it's normal behavior. The MAP sensor tested 4.9ish with the engine off and 1.80 at idle. The TPS tested perfectly, 0.54 volts at close and 4.54 at WOT with a smooth increase/decrease. I've also tried disconnecting the EGR solenoid, it threw a code but no change in engine operation.

    The fuel injectors seemed to spray evenly compared to each other. It was a little hard to tell because I was loosing light. It's a trade off here, plenty of light along with plenty of heat, or fading light with bearable temperatures. I will check again tomorrow in the morning. However, I did notice that while the injectors sprayed a fairly uniform cone pattern, there was also some extra spittle outside of the cone at idle. I don't know if this is normal for a TBI setup or not. I'm used to tightly controlled MPI, and if this were one of those type of injectors I'd say the injectors are spraying a little wild/loose.

    The distributor is plenty tight. All of the vacuum lines are in surprisingly good condition. I will trace them all from end to end tomorrow just to be sure, and test them with carb cleaner as well at the base of the throttle body.

    I was planning on changing the cap and rotor because I don't like that crusty stuff that was on the inside contacts, even though the cap looks newish. I'll go ahead and do the plugs too. The wires look nearly new and show absolutely no cracking or separation. They're even clean, so I'll leave them for now. Is an Airtex/Wells 1A7 a bad choice for a distributor cap? It's absurdly cheap ($5.20 w/rotor) on Rockauto.com. Otherwise I'll probably skip the in betweens and go for an MSD for $30, unless that's a waste. I haven't dealt with a distributor since my first car, and then I just got the cheapest thing I could get my hands on so I could continue driving to and from school.

    I will work on checking the fuel pressure once I figure out where the timing is. Oh yeah, one last thing, I watched the throttle body closely this time and it will pop flame out of either or both sides of the throttle body, if that means anything. I don't know if the intake manifold is divided from the fuel injectors down or not. If so, I suppose that would mean it's not a sticking valve causing the flame.

    And just think, once I get the engine running right I get to diagnose the four wheel drive and the A/C!
     
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  6. mowfixer

    mowfixer Full Member

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    #6 mowfixer, Jun 20, 2015
    Last edited: Jun 20, 2015
    Its possible the fuel pressure regulator in the TBI could be bad but do not know if that would cause back fire. If you try everything else you may want to run a compression test. Backfiring is one of the symptoms I battled with for weeks and after trying all the stuff you already did I decided to do a compression test and found a blown head gasket between two cylinders. Backfiring could also be caused by burned valves. If intake valve is not seating properly when cylinder fires could back fire up the intake.

    Also after running engine and after shutting down, look inside TBI to see if fuel is dripping from one of the injectors. You could have leaking injectors.

    If you have access to vacuum gauge you might want to see if engine vacuum is normal. Might show vacuum leak or burned valves and other things.

    Try everything all the experts said here and for sure check timing but if that does not help, compression test would be next.
     
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  7. MrMarty51

    MrMarty51 Full Member

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    I do know that bad plug wires can cause all of these symptoms, especially a bad coil wire, along with, a "going bad" ignition control module{ICM}.
    You can get Your module tested for free, at most any reliable parts stores.
    Recheck Your TPS, make sure that it will not glitch out. I will sometimes use the "DIODE" test mode on the multimeter for checking the TPS. Also, even better, If Yo have access to an Oscilloscope, that will be even better than a multimeter, unless Your meter has an oscilloscope function.
    Running rich, like a stuck choke always leads Me to an O2 sensor going bad. I don`t know why, it just does.
     
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  8. MrMarty51

    MrMarty51 Full Member

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    OH, Welcome to the Forum D.L.H.
     
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  9. mowfixer

    mowfixer Full Member

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    Trace out plug wires and make sure someone does not have them cross wired too.
     
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  10. MrMarty51

    MrMarty51 Full Member

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    An excellent suggestion. Also, plug wires can cross fire if they are getting to the time of their life, :idea: like Me, breaking down. :rofl: :rofl: :rofl:
     
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